The present invention relates to a common bus-type transmission system of a local area network (LAN) and, more particularly, a driver isolating circuit for a common bus-type transmission system such that when a driver of a transceiver connected to a bus short-circuits, the driver isolating circuit permits the driver to be automatically isolated from the bus.
FIG. 2 shows a block diagram of a typical common bus-type transmission system of a local area network (LAN). It comprises a bus 2 composed of a coaxial cable and a plurality of stations 4 and transceivers 1. Each of the plurality of stations 4 is coupled to the bus 2 through each of the plurality of transceivers 1 which acts as the interface for each statin. Every transceiver 1 comprises a receiver 3 and a driver 5 connected in parallel with the bus 2.
FIG. 3 shows a block diagram of the conventional transceiver 1. The driver 5 comprises a transistor 6 as a switching element, a diode 7 connected between the collector of the transistor 6 and the bus 2, and a resistance 8 for applying a reverse-bias to the diode 7. When transmission data signals from the station 4 are inputted into the base of the transistor 6, the collector and the emitter of the transistor 6 become conductive so that currents can flow into the bus 2 to thereby induce negative voltage signals corresponding to the transmission data signals onto the bus 2. The negative voltage signals can be forwarded to the bus 2 as forward data signals in the form of the negative logic given by reversing the transmission data signals from the station 4.
It is now assumed that the transistor 6 as the switching element short-circuits due to lightning for example and the driver 5 of the troubled transistor 6 is positioned at A of FIG. 2. When the transmission data signals are inputted into the transistor 6 of the driver 5 positioned at N of FIG. 2, the transistor 6 becomes conductive. However, since negative currents have already flown from the short-circuiting transistor 6 of the driver 5 at the A position into the bus 2, the short-circuiting current and the transmission data signals are superposed on the bus 2. The correct transmission data signals from the driver 5 at the N position cannot be detected.
In particular, when the maximum transmission delay time (a time necessary for the signal transmission between the furthest two stations 4) is sufficiently greater than the bit rate, a reflection obstacle occurs on the bus 2 that causes echo noises.
If the diode 7 short-circuits in the driver 5, a similar problem may arise. Although the OFF resistance of the transistor 6 is set comparatively high, the resistance 8 for voltage drop is inserted in parallel with the bus 2, so that the reflections obstacle occurs also on the bus 2 to cause the echo noises.
Thus, the short-circuiting of either the switching element or the diode in one of the drivers damages the data transmission between the all stations in the transmission system.